1. Field of the Invention
This invention relates generally to the blow molding of containers, and is more particularly directed to the blow molding of completely finished hollow containers of molecularly oriented plastic material.
2. Description of the Prior Art
Blow molded molecularly oriented hollow containers possess improved physical characteristics, such as increased strength and transparency. Generally, in the technique for blow molding oriented containers, a cooled tubular preform is heated to a suitable orientation temperature, stretched along the longitudinal axis of the preform to orient the material axially and then blown to the final configuration of the container within a mold cavity, orienting the material transversely.
Various processes and apparatuses exist in the prior art for the blow molding of containers of molecularly oriented material. For example, techniques exist in which a continuous tubing of thermoplastic material is extruded, cooled, reheated to a suitable orientation temperature and blow molded to form a series of hollow containers connected by surplus material. Blow molding may be achieved by sequentially closing cooperating mold halves onto the tubing as it is removed from a reheating oven and forming the container as the mold halves and the tubing move away from the oven. Longitudinal stretching of the material is achieved by moving the mold halves at a speed greater than the speed with which the tubing is being extruded. Examples of this technique are disclosed in U.S. Pat. Nos. 3,798,295, issued to R. MacDuff; 3,751,542, issued to J. N. Hall; and 3,288,317, issued to S. E. Wiley.
Blow molding of containers from a continuous tubing provides good control of the temperature conditions of the tube material which is necessary for good results, and provides a high speed and consequently a high production rate molding technique. However, since the containers are joined together in a continuous string, additional finishing steps are required to separate the containers from the surplus material and to provide an opening in the container neck.
Another general technique for blow molding hollow containers of molecularly oriented material involves cutting short lengths from an extruded tube which has been cooled. The tube lengths or preforms are then heated to a suitable orientation temperature for the material and blow molded. The blow molding may be achieved in different ways, one of which is to place the heated preforms within a mold, forming the neck region and closure threads, if provided, at one end of the preform, stretching the preform either by pulling the preform or by extension of a stretching rod placed within the preform, and then molding the container within the mold cavity. Examples of prior art techniques in which the oriented containers are blow molded in a single molding process from heated preforms are disclosed in U.S. Pat. Nos. 4,106,886, 4,011,039, 3,982,872, 3,896,200, and 3,525,123. Other examples of this technique are disclosed in U.S. Pat. Nos. 3,899,279, 3,872,203, 3,852,398, and 3,765,813.
The blow molding also may be achieved in a two stage process in which the heated preform is placed within a first or a pre-form, mold wherein the neck region and thread closures, if provided, are molded at one end of the preform, and the remainder of the preform is given a shape which resembles the ultimate container configuration. The molded preform is then transferred to a second blow molder in which the container is molded to its final shape.
In the foregoing techniques involving blow molding from heated preforms in which the container neck portions are molded and then the remainder of the parisons are stretched and blow molded, the resulting containers have oriented material in the body but the finished neck does not have oriented material. The neck region, therefore, lacks the clarity and strength that is partially obtainable in a container having at least axially-oriented material. To overcome this problem, processes have been developed which provide orientation of the material in the container neck region. Examples of such processes are disclosed in U.S. Pat. Nos. 3,651,186, issued to J. N. Hall, and 4,065,355 and 4,116,607, issued to R. W. LeGrand in which the preforms are stretched prior to the neck forming and blow molding of the container, thus resulting in orientation of the material in the neck region.
The primary advantage in working with individual preforms is that it is possible to produce a substantially completely finished molded container requiring minimum additional finishing. However, in the techniques known in the prior art the close temperature control over the thermoplastic material and the high production capabilities possible with the continuously-extruded tube technique are not as easily achieved with individual preforms. Additionally, provisions must be made for handling the preforms between the heater and the blow molder, and the above noted patents illustrate different proposals for accomplishing this handling.
Prior art attempts have been made to combine the best features of the continuous tube and individual preform techniques. For example in U.S. Pat. Nos. Re. 29,065, 4,120,634 and 3,806,587, processes used in the blow molding of non-oriented thermoplastic material have been adapted to the production of containers from oriented material. Generally, these techniques involve the continuous extrusion of the thermoplastic material, the severing of lengths of the extrudate which is immediately placed in a blow molder, either a pre-form or a final form molder, and the subsequent achievement of the longitudinal stretching of the material. The apparatuses utilized in these techniques, however, have been complex because of the requirement to thermally condition and to control the temperature of the extruded material after it has been severed and during the blow molding process.